That ability is "When the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost." (4/14/2023) As a Siege enters the battlefield, its controller chooses an opponent to be its protector
You can't choose to enlist a creature later
However, despite the lack of dragon cards, it has plenty of potential since it can repeat this effect as long as you keep drawing dragons
As a result, rather than flashing back a Dread Return and triggering Bridge from Below, the new Dredgevine build attempted to fill the graveyard with Vengevines, cast a couple of Gravecrawlers or Skaab Ruinators in the same turn, and then return a bunch of hasty Vengevines to the battlefield and hopefully kill the opponent with a big hit of surprise damage
One for the 1v1 tables, for sure, this is quietly parked in a lot of Pioneer sideboards
Treasure Cruise Treasure Cruise offers an excellent rate for drawing cards